Fix files external amazon s3

fixes leftover issues found in https://github.com/owncloud/core/issues/11280
- storage id now based on bucket name
- some performance improvements
- better handling of stat for root folder
- log exceptions
- minor typos

cc @icewind1991 @DeepDiver1975 @th3fallen 

needs forward port to master
yes, the directory tests pass and it even survived manual testing. Also see https://github.com/owncloud/core/pull/10508#issuecomment-52596373 and the following comments.